Optical receiver for regeneration of optical signal

ABSTRACT

An optical receiver includes: a converting unit that converts an optical signal into an electrical signal; an amplifying unit that amplifies the electrical signal; a regenerating unit that regenerates the amplified electrical signal; a correcting unit that performs correction of an error included in the regenerated electrical signal; a monitoring unit that performs monitoring of an optical current flowing through the converting unit; and a control unit that calculates a decision threshold based on a result of the correction and a result of the monitoring.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2005-296535, filed on Oct. 11,2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical receiver that regeneratesdata from an optical signal based on an optimal decision threshold thatis set dynamically according to the receiving power of the opticalsignal.

2. Description of the Related Art

With the popularization of the Internet in recent years, data traffic incommunication networks has been significantly increasing. To cope withthe increase of data traffic, an ultra-broadband photonic networkemploying a dense wavelength division multiplexing (DWDM) technology hasbeen developed. An ultra-long-haul data communication can be performedwith DWDM transmission, which uses an optical fiber including severaltens of wavelength channels and a plurality of optical amplifiersconnected in cascade on the optical fiber. In such ultra-long-haul datacommunication, however, the interference between wavelength channelssignificantly increases and the optical signal to noise ratio (OSNR) isseriously deteriorated due to optical noise from the optical amplifiers.Especially, data error due to the optical noise has become a bottleneckfor DWDM transmission because it cannot be prevented by improving thesensitivity of an optical receiver. Therefore, to overcome this opticalnoise bottleneck an improvement of the error correction technologyperformed in the optical receiver is strongly needed.

If the optical receiver corrects the data error using forward errorcorrection (FEC), a bit error rate (BER) of the optical receiver can beobtained from a result of the error correction. On the other hand, thereceiving characteristics of the optical receiver can be improved byoptimizing its decision threshold that varies depending on the OSNR or astate of chromatic dispersion due to long-haul transmission. Therefore,the performance of the optical receiver can be improved by performing afeedback control based on the BER and by adjusting the decisionthreshold to the optimal level.

FIG. 17 is a block diagram of a conventional optical receiver for DWDMtransmission. As shown in FIG. 17, an optical receiver 1 includes aphotodiode (PD) 2, a trans-impedance amplifier (TIA) functioning as apreamplifier 3, a variable-gain amplifier 4, a gain-control amplifier 5,a clock/data recovery (CDR) 6, a forward error correction (FEC) unit 7,a controller 8, and a digital-to-analog converter (DAC) 9.

The PD 2 converts an optical input signal into an electrical signal. Thepreamplifier 3, the variable-gain amplifier 4, and the gain-controlamplifier 5 perform reshaping of the electrical signal. The CDR 6performs regeneration and retiming of the reshaped electrical signal.The FEC 7, the controller 8, and the DAC 9 are provided to adjust thedecision threshold according to the amplitude of the reshaped electricalsignal as shown in FIG. 18 (see, for example, Japanese PatentApplication Laid-Open No. H2-288640).

However, the optical receiver 1 needs large circuit size and its controlbecomes complicated because it has to perform variable-gain control tokeep constant reshaped electrical signal. Furthermore, the gain of thepreamplifier 3 needs to be small to prevent saturation of amplitude whenthe input power of optical signal increases, thereby making it difficultto improve the sensitivity of the optical receiver 1.

On the other hand, another optical receiver achieving high sensitivitywith a simple configuration has also been suggested. The opticalreceiver includes a high-gain limiting amplifier, and a direct current(DC) feedback circuit for controlling the DC level of the positivesignal and the negative signal output from the limiting amplifier. Thesensitivity of the optical receiver can be improved by increasing thegain of the preamplifier, while reducing the circuit size of the opticalreceiver.

In such an optical receiver, however, the relation between the decisionthreshold of optical receiver and a feed-backed threshold control signalfrom an forward error correction (FEC) unit is not unique, because thecondition of signal in the optical receiver greatly differs dependingon, for example, the receiving power of the signal. The limitingamplifier performs a complex operation in the DC feedback control.Specifically, as long as the amplitude of an input signal is less thanpredetermined limiting amplitude, the limiting amplifier performs alinear operation and linearly amplifies the input signal. On the otherhand, when the amplitude of the input signal reaches the limitingamplitude, the limiting amplifier performs a limiting operation andextracts a part of the input signal near cross points. The wide dynamicrange of the receiving power makes it difficult to set an appropriatedecision threshold, using the threshold control signal, for respectiveinput power. As a result, a sufficient error correction cannot beachieved.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the aboveproblems in the conventional technology.

An optical receiver according to an aspect of the present inventionincludes: a converting unit that converts an optical signal into anelectrical signal; an amplifying unit that amplifies the electricalsignal; a regenerating unit that regenerates the electrical signalamplified by the amplifying unit; a correcting unit that performscorrection of an error included in the electrical signal regenerated bythe regenerating unit; a monitoring unit that performs monitoring of anphoto current flowing through the converting unit; and a control unitthat calculates a decision threshold based on a result of the correctionand a result of the monitoring.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical receiver according to a firstembodiment of the present invention;

FIG. 2 is a schematic illustrating an operation of the optical receivershown in FIG. 1;

FIGS. 3 to 6 are waveform diagrams illustrating the output amplitude ofa limiting amplifier shown in FIG. 1;

FIG. 7 is a flowchart of a decision threshold setting process accordingto the first embodiment;

FIG. 8 is a block diagram of an optical receiver according to a secondembodiment of the present invention;

FIG. 9 is a block diagram of an optical receiver according to a thirdembodiment of the present invention;

FIG. 10 is a block diagram of an optical receiver according to a fourthembodiment of the present invention;

FIG. 11 is a block diagram of an optical receiver according to a fifthembodiment of the present invention;

FIG. 12 is a block diagram of an optical receiver according to a sixthembodiment of the present invention;

FIG. 13 is a block diagram of an optical receiver according to a seventhembodiment of the present invention;

FIG. 14 is a block diagram of an optical receiver according to an eighthembodiment of the present invention;

FIG. 15 is a block diagram of an optical receiver according to a ninthembodiment of the present invention;

FIG. 16 is a flowchart of a decision threshold setting process accordingto the ninth embodiment;

FIG. 17 is a block diagram of a conventional optical receiver; and

FIG. 18 is a waveform diagram illustrating the output amplitude of theconventional optical receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention will be explained indetail with reference to the accompanying drawings.

FIG. 1 is a block diagram of an optical receiver according to a firstembodiment of the present invention. An optical receiver 10 includes apower monitor 11, a photodiode (PD) 12, a preamplifier 13, a limitingamplifier 14, a direct current (DC) feedback amplifier 15, a clock/datarecovery (CDR) 16, a forward error correction (FEC) unit 17, and acontroller 18.

The PD 12 converts an optical input signal into an electrical signal.The preamplifier 13 and the limiting amplifier 14 amplify the electricalsignal. An output signal from the preamplifier 13 is input to one of theinput terminals of the limiting amplifier 14. The DC feedback amplifier15 feedbacks an output signal from the limiting amplifier 14 back to theother input terminal of the limiting amplifier 14. Thus, the DC feedbackamplifier 15 controls the DC level of the positive signal and thenegative signal output from the limiting amplifier 14. The CDR 16regenerates and retimes the output signal from the limiting amplifier14.

The FEC 17 corrects data error included in the regenerated signal. Thepower monitor 11 monitors a photo current flowing through the PD 12. Thecontroller 18 calculates an optimal decision threshold according to thereceiving power and the bit error rate. Specifically, the controller 18calculates the optimal decision threshold based on a monitor signal fromthe power monitor 11, which corresponding to the monitored receptionpower, and a threshold control signal from the FEC 17, whichcorresponding to the bit error rate. The calculated decision thresholdis converted into an analog signal in the controller 18, and is set tothe DC feedback amplifier 15.

FIG. 2 is a schematic illustrating an operation of the optical receiver10. FIGS. 3 and 4 are waveform diagrams illustrating the outputamplitude of the limiting amplifier 14 performing the linear operationwith the decision threshold being set at 50% and 30%, respectively.FIGS. 5 and 6 are waveform diagrams illustrating the output amplitude ofthe limiting amplifier 14 performing the limiting operation with thedecision threshold being set at 50% and 30%, respectively. The abovedecision thresholds (%) are normalized with respect to the signalamplitude.

As shown in FIGS. 3 to 6, the limiting amplifier 14 performs the linearoperation and the limiting operation. In the linear operation, thedecision threshold is changed in proportion to the reception power asshown in FIG. 2 because the signal level of the positive signal and thenegative signal changes due to the DC feedback control. On the otherhand, in the limiting operation, the signal level does not change butthe pulse width of the signal changes according to the rising edgetiming and the falling edge timing of the signal. Therefore, as long asthe rising and falling timings are stable in the signal, the decisionthreshold is kept substantially constant in the limiting operation asshown in FIG. 2.

The controller 18 calculates an optimal decision threshold based on theabove operations of the limiting amplifier 14. The DC feedback amplifier15 controls the DC level of the feedback signal to the limitingamplifier 14 based on the decision threshold set by the controller 18,to control the DC level of the positive signal and the negative signaloutput from the limiting amplifier 14.

FIG. 7 is a flowchart of a decision threshold setting process performedby the controller 18. The controller 18 receives the monitor signalindicating the receiving power of an optical signal from the powermonitor 11, and sets an initial value of the decision threshold (stepS1). Then, the controller 18 calculates an initial value of the errorrate based on the initial value of the decision threshold and thethreshold control signal from the FEC 17 (step S2). The controller 18determines whether the error rate satisfies a predetermined condition(step S3). When the error rate satisfies the condition (“YES” at stepS3), the process is completed.

On the other hand, when the error rate does not satisfy the condition(“NO” at step S3), the controller 18 receives updated monitor signalfrom the power monitor 11, and changes the decision threshold (step S4).Then, the controller 18 calculates the error rate (step S5), anddetermines whether the error rate satisfies the condition (step S6).When the error rate does not satisfy the condition (“NO” at step S6),the process returns to step S4. The process from step S4 to step S6 isrepeated until an error rate that satisfies the condition is obtained.When the error rate satisfies the condition (“YES” at step S6), theprocess is completed.

FIG. 8 is a block diagram of an optical receiver according to a secondembodiment of the present invention. An optical receiver 20 shown inFIG. 8 performs a DC feedback control different from the DC feedbackcontrol explained in the first embodiment. Specifically, the opticalreceiver 20 includes a DC feedback amplifier 25 instead of the DCfeedback amplifier 15 shown in FIG. 1. The output signals from thelimiting amplifier 14 are input to the DC feedback amplifier 25. Theoutput signal from the DC feedback amplifier 25 controls a currentsource 22 connected to the PD 12 and the preamplifier 13.

In a similar manner as in the first embodiment, the decision thresholdcalculated by the controller 18 is set in the DC feedback amplifier 25.The output signal from the preamplifier 13 is input to one of the inputterminals of the limiting amplifier 14 as it is, and also input to theother input terminal through a low pass filter (LPF) 21 that extractsthe DC level of the output signal of preamplifier.

The DC feedback amplifier 25 performs a DC feedback control based on thedecision threshold set by the controller 18, to control the DC level ofthe positive signal and the negative signal that are output from thepreamplifier 13 and input to the limiting amplifier 14.

FIG. 9 is a block diagram of an optical receiver according to a thirdembodiment of the present invention. An optical receiver 30 shown inFIG. 9 performs a DC feedback control different from the DC feedbackcontrol explained in the second embodiment. Specifically, the opticalreceiver 30 includes a DC feedback amplifier 35 instead of the DCfeedback amplifier 25 shown in FIG. 8. The output signal from thepreamplifier 13 is input to the DC feedback amplifier 35. The outputsignal from the DC feedback amplifier 35 controls the current source 22.

In a similar manner as in the second embodiment, the decision thresholdcalculated by the controller 18 is set in the DC feedback amplifier 35.However, in the third embodiment, the output signal from thepreamplifier 13 is subjected to a feedback control performed by the DCfeedback amplifier 35, to control the DC level of the positive signaland the negative signal to be input to the limiting amplifier 14.

FIG. 10 is a block diagram of an optical receiver according to a fourthembodiment of the present invention. An optical receiver 40 shown inFIG. 10 controls, instead of performing the DC feedback control, a DClevel of the output signal from the limiting amplifier 14 directly basedon the decision threshold calculated the controller 18. The limitingamplifier 14 and the CDR 16 are AC-coupled via capacitors 41 and 42, andthe decision threshold calculated by the controller 18 is input to oneof the input terminals of the CDR 16 by an adder 43.

FIG. 11 is a block diagram of an optical receiver according to a fifthembodiment of the present invention. The configuration of an opticalreceiver 50 shown in FIG. 11 is similar to that of the optical receiver40 according to the fourth embodiment (see FIG. 10). However, unlike theoptical receiver 40, the optical receiver 50 performs the same DCfeedback control as that of the first embodiment (see FIG. 1).Specifically, the DC feedback amplifier 15 of the optical receiver 50feeds back the output signal from the limiting amplifier 14 to one ofthe input terminals of the limiting amplifier 14. However, the decisionthreshold calculated by the controller 18 is not input to the DCfeedback amplifier 15.

FIG. 12 is a block diagram of an optical receiver according to a sixthembodiment of the present invention. The configuration of an opticalreceiver 60 shown in FIG. 12 is similar to that of the optical receiver40 according to the fourth embodiment (see FIG. 10). However, unlike theoptical receiver 40, the optical receiver 60 performs the same DCfeedback control as that of the third embodiment (see FIG. 9).Specifically, the DC feedback amplifier 35 of the optical receiver 60controls the current source 22 connected to the PD 12 and thepreamplifier 13 by inputting the output signal from the preamplifier 13to the current source 22. However, the decision threshold calculated bythe controller 18 is not input to the DC feedback amplifier 35.

FIG. 13 is a block diagram of an optical receiver according to a seventhembodiment of the present invention. The configuration of an opticalreceiver 70 shown in FIG. 13 is same as that of the optical receiver 50according to the fifth embodiment (see FIG. 11). However, in the opticalreceiver 70, the decision threshold calculated by the controller 18 isinput to the DC feedback amplifier 15 as in the optical receiver 10according to the first embodiment (see FIG. 1). In other words, in theoptical receiver 70, the DC level of the positive signal and thenegative signal output from the limiting amplifier 14 is controlled atboth sides of the limiting amplifier 14 (that is, the input side and theoutput side). According to the seventh embodiment, the decisionthreshold can be adjusted appropriately even when the relation betweenthe reception power and the decision threshold is more complicated.

FIG. 14 is a block diagram of an optical receiver according to an eighthembodiment of the present invention. The configuration of an opticalreceiver 80 shown in FIG. 14 is similar to that of the optical receiver10 according to the first embodiment (see FIG. 1), except for includingan analog operating unit 88, such as an operational amplifier, insteadof the controller 18. The analog operating unit 88 performs an analogprocessing to set the decision threshold based on the monitor signal andthe threshold control signal. With the above configuration, the decisionthreshold is output as an analog signal from the analog operating unit88.

FIG. 15 is a block diagram of an optical receiver according to a ninthembodiment of the present invention. The configuration of an opticalreceiver 90 shown in FIG. 15 is similar to that of the optical receiver10 according to the first embodiment (see FIG. 1), except for includinga controller 91, a calculator 92, and a DAC 93 instead of the controller18. The controller 91 generates a normalized threshold control signalbased on the threshold control signal input from the FEC 17. Thecalculator 92 calculates an optimal decision threshold according to thereception power and the error rate. Specifically, the calculator 92calculates the optimal decision threshold based on the normalizedthreshold control signal input from the controller 91 and the monitorsignal input from the power monitor 11. The DAC 93 converts the optimaldecision threshold output from the calculator 92 from digital to analog,and set the decision threshold to the DC feedback amplifier 15.

FIG. 16 is a flowchart of a decision threshold setting process performedby the controller 91 and the calculator 92. The controller 91 sets aninitial value of the normalized threshold (step S1). Then, thecalculator 92 receives the monitor signal from the power monitor 11, andsets an initial value of the decision threshold (step S12). Thecalculator 92 calculates an initial value of the error rate based on theinitial values of the normalized threshold and the decision threshold(step S13), and determines whether the error rate satisfies apredetermined condition (step S14). When the error rate satisfies thecondition (“YES” at step S14), the process is completed.

On the other hand, when the error rate does not satisfy the condition(“NO” at step S14), the controller 91 changes the normalized threshold(step S15). The calculator 92 receives updated monitor signal from thepower monitor 11, and changes the decision threshold (step S16). Thecalculator 92 recalculates the error rate based on the normalizedthreshold and the decision threshold (step S17), and determines whetherthe error rate satisfies the condition (step S18).

When the error rate does not satisfy the condition (“NO” at step S18),the process returns back to step S15, and the process from step S15 tostep S18 is repeated until an error rate that satisfies the condition isobtained. When the error rate satisfies the condition (“YES” at stepS18), the process is completed.

The configuration according to the ninth embodiment is suitable for acase in which the controller 91 and the calculator 92 are separatelyprovided. For example, a module formed by the calculator 92, the DAC 93,and the PD 12 can be mounted on a substrate provided with the controller91. The controller 18 or the analog calculator 88 according to the firstto the eighth embodiments may also be provided as two independentcomponents of the controller and the calculator.

According to the embodiments described above, an optimal decisionthreshold is set according to the receiving power varying in a widerange, thereby improving the performance of the error correctionperformed by an optical receiver. Moreover, a high-quality anderror-free optical transmission can be achieved by applying a high-gainerror correction technology to the highly-sensitive optical receiverwith a limiting amplifier.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. An optical receiver comprising: a converting unit that converts an optical signal into an electrical signal; an amplifying unit that amplifies the electrical signal; a regenerating unit that regenerates the electrical signal amplified by the amplifying unit; a correcting unit that performs correction of an error included in the electrical signal regenerated by the regenerating unit; a monitoring unit that performs monitoring of an optical current flowing through the converting unit; and a control unit that calculates a decision threshold based on a result of the correction and a result of the monitoring.
 2. The optical receiver according to claim 1, further comprising a feedback unit that controls a direct current level of a signal output from the amplifying unit by branching a part of the signal to be fed back to the amplifying unit.
 3. The optical receiver according to claim 2, wherein the decision threshold is input to the feedback unit.
 4. The optical receiver according to claim 1, further comprising: a pre-stage amplifying unit that outputs a signal to the amplifying unit; a supplying unit that supplies a current to the pre-stage amplifying unit; and a feedback unit that controls a direct current level of a signal output from the amplifying unit by branching a part of the signal output from the pre-stage amplifying unit to be input to the supplying unit.
 5. The optical receiver according to claim 4, wherein the decision threshold is input to the feedback unit.
 6. The optical receiver according to claim 1, wherein the decision threshold is input to the regenerating unit.
 7. The optical receiver according to claim 1, wherein the control unit includes an analog operating unit.
 8. The optical receiver according to claim 1, wherein the control unit includes a controller that outputs a normalized threshold corresponding to the result of the correction; and a calculator that calculates the decision threshold based on the result of the monitoring and the normalized threshold.
 9. The optical receiver according to claim 1, wherein the amplifying unit includes a limiting amplifier. 